Method for forming integrated circuit gate conductors from dual layers of polysilicon

ABSTRACT

A method for fabricating an integrated circuit is presented wherein a first polysilicon layer dielectrically spaced above a semiconductor substrate is provided. The semiconductor substrate contains a first active region and a second active region. A first dopant is selectively introduced into the portion of the first polysilicon layer above the second active region. A second polysilicon layer may then be formed upon the first polysilicon layer and above the first active region and the second active region. A second dopant may be selectively introduced into a portion of the second polysilicon layer above the first active region. The portion of the second polysilicon layer above the first active region and the portion of the first polysilicon layer above the first active region may be patterned to form a first gate structure within the first active region. The portion of the second polysilicon layer above the second active region and the portion of the first polysilicon layer above the second active region may be patterned to form a second gate structure within the second active region.

This is a Division of application Ser. No. 09/237,773, filed Jan. 26,1999, U.S. Pat. No. 6,137,145.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor processing and, moreparticularly, to a method for forming CMOS integrated circuit gateconductors having reduced susceptibility to polysilicon depletion andboron penetration.

2. Description of the Related Art

Fabrication of a metal oxide semiconductor field-effect transistor(“MOSFET”) device is well known. MOSFETs are generally manufactured byplacing an undoped polycrystalline silicon (“polysilicon”) layer over arelatively thin silicon dioxide (“oxide”) layer. The polysilicon layerand the oxide layer are then patterned to form a gate conductor arrangedupon a gate oxide with source/drain regions adjacent to and on oppositesides of the gate conductor. The gate conductor may be used toself-align impurities forwarded into the substrate on opposite sides ofthe gate conductor. The gate conductor and source/drain regions are thenconcurrently implanted with a dopant species. If the dopant species usedis n-type, then the resulting MOSFET is an NMOS (“n-channel”) transistordevice. Conversely, if the dopant species is p-type, then the resultingMOSFET is a PMOS (“p-channel”) transistor device. Integrated circuitstypically use either n-channel devices exclusively, p-channel devicesexclusively, or a combination of both on a single substrate. Thecombination of an n-channel device and a p-channel device on a singlesubstrate is termed a complementary MOS (“CMOS”) device.

CMOS circuits offer numerous performance, reliability, design, and costadvantages over NMOS or PMOS circuits, and have become the dominantintegrated circuit technology. One basic process for forming transistorsfor CMOS circuits (hereinafter “the basic CMOS process”) only requiresslight modification of the general technique for forming MOSFETsdescribed above. In the basic CMOS process, a thin oxide layer is firstformed upon a silicon substrate. The silicon substrate contains twoactive regions laterally separated by a field region. The field regionincludes an isolation structure, which may be formed by trench isolationor local oxidation of silicon (“LOCOS”) techniques. A single layer ofundoped polysilicon is then deposited on the oxide layer. Gatestructures are then formed within the active regions by patterning thelayer of polysilicon and the layer of oxide. The resulting structureseach include a gate conductor formed from the polysilicon layer and agate oxide formed from the oxide layer. One of the active regions,typically the region in which the p-channel device is to be formed, iscovered with a masking layer of photoresist. N-type dopants areconcurrently implanted into the other gate conductor and the adjacentsemiconductor substrate. Such implantation serves both to dope the gateconductor and to form lightly-doped regions (“LDD”) in the siliconsubstrate. Oxide spacers are then formed on the sidewall of theuncovered gate structure. A second implant dose is then forwarded intothe gate structure and the silicon substrate adjacent to the exposedlateral surfaces of the spacers. The second implant is done at a higherimplant energy and dose than the first and creates source/drain regionswithin the silicon substrate. The process is then repeated for thep-channel transistor, except now p-type dopants are implanted.

As implied above, ion implantation is commonly used both for formingjunctions and for doping the gate conductor. An advantage of ionimplantation over older techniques such as solid-state diffusion is thation implantation allows greater control over the dopant profile. Bymodifying the implant dose and energy appropriately, the depth of theimplanted area and the dopant distribution within that area may beprecisely controlled. Because of this property, ion implantation isusually used to form the shallow junctions necessitated by thesub-micron channel lengths common in MOS transistors. Shallow junctionshelp reduce susceptibility to short-channel effects, make the deviceless prone to punchthrough effects, and reduce parasitic capacitance. Byreducing the vertical depth of the junctions, the lateral spread ofdopants beneath the gate structure from these regions is also curtailed.Hence, the degree of reduction in effective channel length resultingfrom dopant migration is lessened. To achieve shallow junctions,relatively low implant energies are used to ensure that the dopants areimplanted close to the upper surface of the semiconductor substrate.

In forming MOS devices with polysilicon gate conductors, the polysiliconfrom which the gate conductor is formed must be doped to lower its sheetresistivity. Dopants should also be implanted sufficiently deep withinthe gate conductor that a large quantity of the dopants diffuse to thebottom of the gate conductor during subsequent heat processing.Accordingly, dopants, particularly heavier n-type atoms such as arsenic,must be given a relatively high implant energy to ensure that theseatoms are implanted sufficiently deep within the gate conductor. On theother hand, dopants that diffuse relatively quickly through polysilicon,such as boron, do not need to be implanted as deeply. Regardless of thediffusion rate through polysilicon of the implanted dopant, the idealimplant energy for doping the gate conductor is often higher than theideal implant energy for forming shallow junctions. Furthermore, thedifferent diffusion rates through polysilicon for n-type dopants (e.g.,arsenic and phosphorus) and p-type dopants (e.g., boron and borondifluoride) may be problematic when attempting to form p-channel andn-channel gate conductors from a single polysilicon layer. Twoconditions that can result in such a situation are polysilicon depletionand boron penetration.

Arsenic is typically used to dope the polysilicon gate and to formjunctions within the silicon substrate for n-channel devices. As statedabove, arsenic is a relatively slow diffuser and will not readilymigrate through polysilicon even during substantial heat treatment. Ifarsenic is not implanted deep into the gate conductor, the arsenicdiffusion that will result from subsequent heat processing may not beenough to cause a sufficient quantity of arsenic to migrate near thepolysilicon/oxide interface. Thus, a lower portion of the polysiliconremains substantially undoped or “depleted”. The undoped lower portionof the gate conductor acts as a high permittivity region thatdeleteriously hinders performance of the transistor by increasing theeffective thickness of the gate oxide. This increase in effective oxidethickness can cause the “turn-off” characteristics of the transistor toincrease beyond acceptable values.

Boron, in the form of boron or boron difluoride ions, is typically usedto dope the polysilicon gate and to form the junctions within thesilicon substrate for p-channel devices. The diffusion rate of boron inpolysilicon is relatively high, so boron implanted into a gate conductorwill migrate significantly during heat processing. Boron penetration canoccur when boron atoms are implanted too deeply into the gate conductor.In such circumstances, the diffusion of these atoms can result in theatoms penetrating the gate oxide. This penetration may reduce thereliability of the gate oxide. Furthermore, boron may even continuethrough the gate oxide into the underlying channel. The presence ofboron in the channel can change the doping concentration in the channel,resulting in threshold voltage shift. Boron penetration into the channelcan also cause other undesirable effects such as an increase in electrontrapping, a decrease in low-field hole mobility, and degradation of thedrive current. Because boron migrating from the lower portions of thegate conductor as a result of uncontrolled channel doping may leave thelower portion of the gate conductor with a less than optimal dopantconcentration, boron penetration can also cause polysilicon depletion.

Since the basic CMOS process forms both the n-channel and p-channel gateconductor from the same layer of polysilicon, both gate conductors willhave the same thickness. While it may be beneficial to increase thethickness of the gate conductors by increasing the thickness of thepolysilicon layer from which they are formed so that boron penetrationis minimized, such an increase would make getting the necessary amountof arsenic to the polysilicon-oxide interface in the n-channel conductoreven more difficult. Likewise, decreasing the thickness of thepolysilicon layer from which the gate conductors are formed would helpensure that arsenic was implanted sufficiently deep in the n-channelconductor, but the reduction in the height of the p-channel gateconductor may increase the probability of boron penetration in thatdevice.

One prior process that attempts to resolve this dilemma starts bydepositing a single layer of polysilicon upon an oxide layer. The oxidelayer overlies a pair of active areas within a silicon substrate. A maskis formed over the portions of the polysilicon layer above the activearea in which the n-channel device will be fabricated, and the uncoveredportions of the polysilicon layer are implanted with a p-type dopantsuch as boron. The previously implanted portions of the polysiliconlayer are now covered with a mask while the undoped portion ofpolysilicon layer overlying the n-channel active area is etched back toa second thickness that is substantially less than the initial thicknessof the polysilicon layer. An n-type dopant such as arsenic is thenimplanted into the unmasked portions of the polysilicon layer. Thepolysilicon layer is then patterned to produce a relatively thickp-doped gate conductor in one active area and a thinner n-doped gateconductor in the other active area. In this manner, the p-doped gateconductor may be made thick enough to prevent boron penetration and then-doped gate conductor may be thin enough to ensure that an adequateamount of n-type dopant is placed at the gate conductor lowerelevations.

Unfortunately, the above-described process has numerous disadvantages.One of these disadvantages arises from the fact that it is difficult toachieve highly accurate thickness control when etching back a layer ofpolysilicon. If the precise thickness of the n-channel gate conductor isnot known, correctly modeling device properties may be difficult. Inaddition, a layer of photoresist will be spun on and developed to definethe pattern for subsequent etching of the p-channel gate conductor andn-channel gate conductor. The difference between the thickness of thepolysilicon layer at the portion from which the p-channel conductor willbe formed and the thickness of the polysilicon at the portion from whichthe n-channel gate conductor will be formed can unfortunately lead to,for example, depth of field problems when exposing the photoresist. Thisand other focus problems may lead to an inaccurate pattern beingtransferred to the resist, and ultimately to the polysilicon layer.Furthermore, it is preferred that etching of the polysilicon layer toform each gate conductor occurs concurrently. The polysilicon layer isthicker, however, in the portions from which the p-channel device willbe formed than in the portions from which the n-channel device will beformed. Consequently, a simultaneous etch of both of these portions willreach the portions of the oxide layer beneath the portions of thepolysilicon layer earlier on the n-doped side of the polysilicon layerthan on the p-doped side. The gate conductor etch process must,unfortunately, be made more complex to resolve this situation.

Therefore, it would be desirable to develop a technique for fabricatingadjacent n-channel and p-channel transistors that have gate conductorsof similar heights but are still substantially resistant to polysilicondepletion and boron migration. It would be further beneficial to achievesuch goals without needing to rely on etchback techniques to control thethickness of the gate conductors. The process should allow dopantimplantation of the gate conductors to be carried out independently ofdopant implantation for junction formation.

SUMMARY OF THE INVENTION

The problems described above are in large part addressed by the improvedintegrated circuit fabrication method and product presented herein. Themethod provides for the formation of adjacent n-channel and p-channeltransistors having gate conductors etched from dual layers ofpolysilicon. Dopants are forwarded into select portions of eachpolysilicon layer after the layer is formed. Because of the ability toscale the total thickness of polysilicon into which each set of dopantsis introduced, the probability of boron penetration and polysilicondepletion is substantially reduced. Furthermore, the present processaffords a substantially planar surface for the patterning of the gateconductors. As such, depth of field problems, as well as any otherpatterning difficulties caused by lack of planarization, are minimized.In the present method, the implants into the polysilicon to render theto-be-formed gate conductors conductive are preferably done such thatjunctions are not concurrently formed. Consequently, the ion energy inany junction formation step may be determined without regard to the needfor doping the gate conductors.

Broadly speaking, a method for fabricating an integrated circuit ispresented wherein a first polysilicon layer is dielectrically spacedabove a semiconductor substrate. The semiconductor substrate contains afirst active region and a second active region. A first dopant isselectively introduced into the portion of the first polysilicon layerabove the second active region. A second polysilicon layer may then beformed upon the first polysilicon layer and above the first activeregion and the second active region. A second dopant may be selectivelyintroduced into a portion of the second polysilicon layer above thefirst active region.

The diffusion length of dopants implanted into the gate conductor mustbe taken into account when attempting to prevent polysilicon depletionand boron penetration. There are four main variables that determine howfar implanted dopants will diffuse after implantation into polysilicon:the implant species, the implant dose, the implant energy, and thethermal budget. The particular properties of an implant species, ofcourse, determine the diffusion constant of that species. The implantdose has a large effect on the concentration distribution of the implantspecies. The implant energy has a strong impact on the depth at whichthe species is implanted. Finally, the total thermal budget of allsubsequent heat processing steps must be accounted for if the finalposition of the implanted dopants is to be known. Thermal budget isoften used to relate the total amount of heat transferred to a wafer bya particular process. In general, the higher the thermal budget of aheat treatment process is, the greater the diffusion length of aparticular dopant brought about that process.

The present process beneficially allows for the thickness of thepolysilicon layer to be scaled to take into account variations in thediffusion length of the implanted dopants caused by changes in theaforementioned variables. Such scaling may be accomplished, in part, bydeposition of polysilicon layers at appropriate thicknesses. Depositionof the polysilicon layers affords greater control over the thickness ofsuch layers than, for example, some prior methods that attempt toachieve similar goals by use of etchback techniques. The first dopant ispreferably a slower diffusing n-type dopant (such as arsenic orphosphorus), and the second dopant is preferably a faster diffusingp-type dopant (such as boron or boron difluoride). In the presentprocess, the slower diffusing dopant is preferably implanted into firstpolysilicon layer while the faster diffusing dopant is preferablyimplanted into the second polysilicon layer. Consequently, the firstpolysilicon layer may be deposited such that the peak concentration ofthe first dopant, subsequent to and during further processing steps,diffuses sufficiently close to the underlying gate oxide to avoid dopantdepletion without a substantial quantity of the first dopant diffusingthrough the gate oxide.

The second polysilicon layer is preferably deposited upon the firstpolysilicon layer after implantation of the first dopant. Thus, thesecond polysilicon layer may be deposited such that the total thicknessof the first and second polysilicon layers allows the peak concentrationof the second dopant, subsequent to and during further processing steps,to diffuse sufficiently close to the underlying gate oxide that dopantdepletion is avoided without a substantial quantity of the second dopantdiffusing through the gate oxide. The process therefore allows thethickness of the first polysilicon layer to be largely determined by thediffusion variables applicable to the first dopant and the thickness ofthe second polysilicon layer to be determined largely by the diffusionvariables applicable to the second dopant.

A semiconductor substrate is also presented. A first gate conductor isarranged upon a first gate dielectric and above the semiconductorsubstrate, and a second gate conductor is arranged upon a second gatedielectric and above the semiconductor substrate. The semiconductorsubstrate may contain a first active region laterally separated from asecond active region by a field region. The first gate conductor may bearranged within the first active region, and the second gate conductormay be arranged within the second active region. Each gate conductorpreferably includes a second polysilicon layer portion arranged upon afirst polysilicon layer portion. The thicknesses of the first gateconductor and the second gate conductor are preferably equal. The firstgate conductor may be doped with a first dopant that has a lowerdiffusion rate through polysilicon than a second dopant with which thesecond gate conductor is doped. The second polysilicon layer portion ofthe second gate conductor is substantially free of implanted dopants.However, dopant ions may later be implanted into the portions of thesemiconductor substrate within the first and second active regionadjacent to the first and second gate conductors to form junctions.Implantation of the semiconductor substrate to form such junctionspreferably results in dopant implantation of the second polysiliconlayer portion of the second gate electrode such that the portion is nolonger substantially free of implanted dopants. The polysilicon gateelectrodes may later be reacted with cobalt or titanium to form a verylow resistance polycide.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a partial cross-sectional view of a semiconductor topography;

FIG. 2 is a partial cross-sectional view of the semiconductortopography, wherein an oxide layer is formed according to an initialprocessing step subsequent to FIG. 1;

FIG. 3 is a partial cross-sectional view of the semiconductortopography, wherein a first polysilicon layer is formed upon the oxidelayer according to a processing step subsequent to FIG. 2;

FIG. 4 is a partial cross-sectional view of the semiconductortopography, wherein a first dopant is selectively introduced into aportion of the first polysilicon layer according to a processing stepsubsequent to FIG. 3;

FIG. 5 is a partial cross-sectional view of the semiconductortopography, wherein a second polysilicon layer is formed upon the firstpolysilicon layer according to a processing step subsequent to FIG. 4;

FIG. 6 is a partial cross-sectional view of the semiconductortopography, wherein a second dopant is selectively introduced into aportion of the second polysilicon layer according to a processing stepsubsequent to FIG. 5;

FIG. 7 is a partial cross-sectional view of the semiconductortopography, wherein a masking layer is stripped from above the secondpolysilicon layer according to a processing step subsequent to FIG. 6;

FIG. 8 is a partial cross-sectional view of the semiconductortopography, wherein a first gate structure and a second gate structureare formed according to a processing step subsequent to FIG. 7; and

FIG. 9 is a partial cross-sectional view of the semiconductortopography, wherein LDD regions, sidewall spacers, and source/drainregions are formed adjacent to each gate structure according to aprocessing step subsequent to FIG. 8.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 shows a partial cross-sectionalview of semiconductor substrate 100. Semiconductor substrate 100preferably comprises an epitaxial layer of lightly doped n-type orp-type single crystal silicon. This lightly-doped epitaxial layer isformed upon a more heavily-doped wafer of single crystal silicon.Isolation structure 102 may be formed in semiconductor substrate 100 tocreate field region 108. Suitable structures for isolation structure 102include a shallow trench isolation structure or an isolation structureformed by LOCOS techniques. Field region 108 electrically isolatesactive regions 106 a and 106 b from one another and possibly from otheractive regions on the substrate. In subsequent process steps, ap-channel transistor will preferably be formed in active region 106 a,and an n-channel transistor will preferably be formed in active area 106b. At least one well (not shown) is preferably formed within the activeregions of semiconductor substrate 100. An n-well may be formed inactive region 106 a, and a p-well may be formed in active region 106 b.

FIG. 2 depicts the formation of dielectric layer 104 upon semiconductorsubstrate 100. Dielectric layer 104 is preferably composed of oxidegrown upon semiconductor substrate 100. Alternately, dielectric layermay be composed of oxide deposited by, for example, chemical vapordeposition (“CVD”) techniques onto semiconductor substrate 100. Whileoxide is a presently preferred material for dielectric layer 104, thislayer may be made of any material capable of being used in a gatedielectric.

FIG. 3 shows the formation of first polysilicon layer 110 upondielectric layer 104. First polysilicon layer 110 is deposited by, forexample, low pressure chemical vapor deposition (“LPCVD”) techniques.Deposition of first polysilicon layer 110 may be undertaken such that alayer about 1500 angstroms thick is formed. The thickness of firstpolysilicon layer 110 is preferably uniform over active areas 106.

FIG. 4 illustrates the selective introduction of first dopant 114 into aportion of first polysilicon layer 110 above active region 106 b. Aspart of this selective introduction, masking layer 112, preferablycomposed of photoresist, may be spun on to first polysilicon layer 110.Masking layer 112 may then be patterned such that the portion of firstpolysilicon layer 110 above active region 106 a is covered by maskinglayer 112. First dopant 114 may then be implanted into the uncoveredportion of first polysilicon layer 110 above active region 106 b. Firstdopant 114 preferably is an n-type dopant with a relatively slowdiffusion rate through polysilicon, such as arsenic. The implant depthof first dopant 114 is preferably such that the thermal budget of allremaining processing steps results in the diffusion of a sufficientquantity of first dopant 114 to the interface between first polysiliconlayer 110 and dielectric layer 104. First dopant 114 should not beimplanted so deeply, however, that the total thermal budget of anysubsequent process steps results in the diffusion of a substantialquantity of first dopant 114 into dielectric layer 104, or further intosemiconductor substrate 100.

Turning now to FIG. 5, masking layer 112 may be stripped and secondpolysilicon layer 116 may then be formed upon first polysilicon layer110. Second polysilicon layer 116 is preferably deposited by, forexample, LPCVD methods. Deposition of second polysilicon layer 116 maybe undertaken such that a layer approximately 500-1000 angstroms thickis formed. Because second dopant 119 is preferably implanted throughboth second polysilicon layer 116 and first polysilicon layer 110, thethickness of second polysilicon layer 116 may be less than the thicknessof first polysilicon layer 110. The ratio between the thickness ofsecond polysilicon layer 116 and the thickness of first polysiliconlayer 110 is preferably about 1:1.5-3.0. In one embodiment, secondpolysilicon layer 116 is about 500-1000 angstroms thick. Secondpolysilicon layer 116 is preferably uniformly thick over active areas106.

FIG. 6 depicts the selective introduction of second dopant 119 into aportion of second polysilicon layer 116 above active region 106 a. Aspart of this selective introduction, masking layer 118, preferablycomposed of photoresist, may be spun on to second polysilicon layer 116.Masking layer 118 may then be patterned such that the portion of secondpolysilicon layer 116 above active region 106 b is covered by maskinglayer 118. Second dopant 119 may then be implanted into the unprotectedportion of second polysilicon layer 116 above active region 106 a.

Second dopant 119 is preferably a p-type dopant with a relatively fastdiffusion rate through polysilicon, such as boron implanted as boron orboron difluoride ions. Depending on the implant energy with which seconddopant 119 is implanted, a considerable quantity of second dopant 119may be implanted into first polysilicon layer 110 as well. The implantdepth of second dopant 119 is preferably such that the total thermalbudget of all remaining processing steps results in the diffusion of asufficient quantity of first dopant 114 to the interface between firstpolysilicon layer 110 and dielectric layer 104. Second dopant 104 shouldnot be implanted so deeply, however, that the thermal budget of thesubsequent process steps results in the diffusion of a substantialquantity of first dopant 114 into dielectric layer 104, or further intosemiconductor substrate 100. Because second dopant 119 preferably has ahigher diffusion rate through polysilicon than first dopant 114, thecombined thickness of second polysilicon layer 116 and first polysiliconlayer 110 gives second dopant 119 more polysilicon to diffuse throughbefore reaching dielectric layer 104. Nevertheless, the implant energywith which second dopant 119 is implanted may be substantially less thanthe implant energy with which first dopant 114 is implanted.

FIG. 7 depicts a processing step in which masking layer 118 is strippedfrom above second polysilicon layer 118. Referring now to FIG. 8, secondpolysilicon layer 116, first polysilicon layer 110, and dielectric layer104 may be patterned to form gate structures within active regions 106.Patterning of the polysilicon layers to form the gate structures ispreferably accomplished by an anisotropic dry etch process. The gatestructure within active area 106 a preferably includes a gate conductor(second polysilicon layer portion 120 a and first polysilicon layerportion 122 a) and a gate oxide (gate dielectric 124 a). Likewise, thegate structure within active area 106 b preferably includes a gateconductor (second polysilicon layer portion 120 b and first polysiliconlayer portion 122 b) and a gate oxide (gate dielectric 124 b). Thethicknesses of each gate conductor are approximately equal;consequently, the heights of each gate structure are about the same.Because second polysilicon layer 116 is preferably masked during theimplantation of second dopant 119, second polysilicon layer portion 120b (formed from second polysilicon layer 116) should be substantiallyfree of implanted dopants at this stage of the process. One or moreannealing steps to repair implant damage may be done before or afterpatterning of the polysilicon layers to form the gate conductors, ifdesired.

FIG. 9 presents the results of subsequent processing steps in which LDDregions 128 are implanted into semiconductor substrate 100, spacers 126are formed upon the sidewalls of the gate structures, and source/drainregions are formed by an implant step aligned to the exposed lateralsurfaces of spacer 126. LDD regions, spacers, and source/drain regionsare preferably formed for the n-channel transistor in active area 106 bbefore the formation of their counterparts to create the p-channeltransistor in active area 106 a. Because the gate conductors wereimplanted with dopants in previous steps, the implant energy for thesejunction formation steps may be determined without consideration of anyneed to dope the gate conductors. Since shallow junctions are generallydesirable in sub-micron MOSFETs, the implant energy used in thesejunction formation steps is preferably less than that used to implantfirst polysilicon layer 110 and second polysilicon layer 116. Heattreatment steps are preferably undertaken to anneal implant damageresulting from the junction formation process. If the implant damagedone by the implantation of first polysilicon layer 110 and secondpolysilicon layer 116 has not yet been annealed, then this damage may berepaired at this point as well. The above-described junction formationflow is a presently preferred technique. Any number of alternate methodsfor forming junctions that would be obvious to one skilled in the arthaving the benefit of this disclosure may be incorporated.

It will be appreciated by those skilled in the art having the benefit ofthis disclosure that this invention is believed to provide a method forforming adjacent, oppositely-doped, transistors within an integratedcircuit. Further modifications and alternative embodiments of variousaspects of the invention will be apparent to those skilled in the art inview of this description. It is intended that the following claims beinterpreted to embrace all such modifications and changes and,accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. A method for fabricating an integrated circuit,comprising: providing a first polysilicon layer dielectrically spacedabove a first active region and a second active region of asemiconductor substrate; selectively introducing a first dopant into aportion of the first polysilicon layer above the second active region;forming a second polysilicon layer upon the first polysilicon layer andabove the first active region and the second active region; andselectively introducing a second dopant into a portion of the secondpolysilicon layer above the first active region.
 2. The method of claim1, wherein said selectively introducing a first dopant comprises:masking a portion of the first polysilicon layer above the first activeregion; and implanting the first dopant into the portion of the firstpolysilicon layer above the second active region, wherein the firstdopant is an n-type dopant.
 3. The method of claim 1, wherein saidselectively introducing a second dopant comprises: masking a portion ofthe second polysilicon layer above the second active region; andimplanting the second dopant into the portion of the second polysiliconlayer above the first active region, wherein the first dopant is ap-type dopant.
 4. The method of claim 1, further comprising subsequentto said selectively introducing a second dopant: patterning the portionof the second polysilicon layer above the first active area and theportion of the first polysilicon layer above the first active area suchthat a first gate conductor is formed; and patterning the portion of thesecond polysilicon layer above the second active area and the portion ofthe first polysilicon layer above the second active area such that asecond gate conductor is formed.
 5. The method of claim 1, wherein saidforming a second polysilicon layer comprises depositing the secondpolysilicon layer such that a thickness of the second polysilicon layeris less than a thickness of the first polysilicon layer.
 6. The methodof claim 5, wherein said forming a second polysilicon layer comprisesdepositing the second polysilicon layer such that a ratio between athickness of the second polysilicon layer and a thickness of the firstpolysilicon layer is about 1:1.5-3.0.
 7. The method of claim 1, whereina diffusion rate through polysilicon of the first dopant is less than adiffusion rate through polysilicon of the second dopant.
 8. A method forfabricating an integrated circuit, comprising: providing a semiconductorsubstrate comprising a first active region and a second active region,the first active region and second active region being laterallyseparated by a field region; forming an oxide layer upon thesemiconductor substrate; depositing a first polysilicon layer upon theoxide layer; while masking a portion of the first polysilicon layerabove the first active region, implanting a first quantity of a firstdopant into a portion of the first polysilicon layer above the secondactive region; depositing a second polysilicon layer upon the oxidelayer; while masking a portion of the second polysilicon layer above thesecond active region, implanting a first quantity of a second dopantinto a portion of the second polysilicon layer above the first activeregion; subsequent to said implanting a first quantity of a seconddopant, forming a first gate structure from a portion of the secondpolysilicon layer above the first active area and a portion of the firstpolysilicon layer above the first active area, wherein the first gatestructure comprises a first gate conductor and a first gate oxide; andsubsequent to said implanting a first quantity of a first dopant,forming a second gate structure from a portion of the second polysiliconlayer above the second active area and a portion of the firstpolysilicon layer above the second active area, wherein the second gateconductor comprises a second gate conductor and a second gate oxide. 9.The method of claim 8, wherein the first dopant is n-type, and whereinthe second dopant is p-type.
 10. The method of claim 9, wherein thefirst dopant is arsenic or phosphorus, and wherein the second dopant isboron or boron difluoride.
 11. The method of claim 8, wherein a heightof the first gate structure and a height of the second gate structureare approximately equal.
 12. The method of claim 8, wherein a peakconcentration of the first dopant is a first distance from a bottom ofthe second gate conductor, and wherein said depositing a firstpolysilicon layer comprises depositing the first polysilicon layer at athickness such that the peak concentration of the first dopant,subsequent to and during further processing steps, diffuses sufficientlyclose to the second gate oxide to avoid dopant depletion in a bottomportion of the second gate conductor without a substantial portion ofthe first quantity of the first dopant diffusing into the second gateoxide.
 13. The method of claim 8, wherein a peak concentration of thesecond dopant is a first distance from a bottom of the first gateconductor, and wherein said depositing a second polysilicon layercomprises depositing the second polysilicon layer at a thickness suchthat the peak concentration of the second dopant, subsequent to andduring further processing steps, diffuses sufficiently close to thefirst gate oxide to avoid dopant depletion in a bottom portion of thefirst gate conductor without a substantial portion of the first quantityof the second dopant diffusing into the first gate oxide.
 14. The methodof claim 8, further comprising implanting a second quantity of the firstdopant into the second active region of the semiconductor substrate andthe first gate conductor such that junctions are formed within thesecond active region of the semiconductor substrate laterally adjacentto the second gate structure, and wherein said implanting a secondquantity of the first dopant is at a lesser implant energy than saidimplanting a first quantity of a first dopant.
 15. The method of claim8, further comprising implanting a second quantity of the second dopantinto the first active region of the semiconductor substrate and thefirst gate conductor such that junctions are formed within the firstactive region of the semiconductor substrate laterally adjacent to thefirst gate structure, and wherein said implanting a second quantity ofthe second dopant is at a lesser implant energy than said implanting afirst quantity of a second dopant.